Differential multilevel modulated optical signal receiver apparatus

ABSTRACT

A first optical splitter splits an input optical signal and outputs it to first and second optical paths. A second optical splitter outputs the optical signal from the first optical path to third and fourth optical paths. A third optical splitter outputs the optical signal from the second optical path to fifth and sixth optical paths. In the second optical path, 1-symbol delay element and π/4 phase shifter element are configured. In the fourth optical path, π/2 phase shifter element is configured. First and second adjuster circuits adjust the optical path length of the second and the fourth optical paths, respectively, by temperature control. A first optical coupler couples optical signals transmitted via the third and the fifth optical paths. A second optical coupler couples optical signals transmitted via the fourth and the sixth optical paths. Photodetectors convert the optical signals from the optical couplers into electrical signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a differential multilevel opticalsignal receiver apparatus for receiving an optical signal modulated bydifferential multilevel modulation.

2. Description of the Related Art

As a technology for transmitting signals in an optical transmissionsystem, phase modulation has been put to practical use widely. In phasemodulation, data is transmitted by shifting the phase of a carrier wavein accordance with the transmitted data. In Quadrature PhaseShift Keying(QPSK), for example, “θ”, “θ+π/2”, “θ+π” and “θ+3π/2” are assigned toeach symbol comprising 2-bit data, “00”, “01”, “11” and “10”,respectively. Here, “θ” is an arbitrary phase. A receiver apparatus canregenerate the transmitted data by detecting the phase of the receivedsignal.

When increasing the transmission speed or distance of a transmissionpath, deterioration of an optical S/N ratio becomes a problem in thereceiver apparatus. In recent years, research and development ofdifferential multilevel optical modulation has advanced as a modulationmethod which enables the improvement of receiver sensitivity. In thisdescription, an explanation is provided of an example of DifferentialQuadrature Phase Shift Keying (DQPSK) modulation representing themodulation. In DQPSK modulation, the phase of a carrier wave (“θ”“θ+π/2”, “θ+π” or “θ+3π/2”) is determined in accordance with a“difference” between a symbol value transmitted previously and a symbolvalue to be transmitted next. Therefore, when demodulating the DQPSKsignal in the receiver apparatus, a phase difference between the twoconsecutive symbols is detected.

FIG. 1 is a diagram describing an example of a conventional DQPSKoptical receiver apparatus. In FIG. 1, an optical splitter 101 splits aninput optical signal and guides the split signals to interferometers 110and 120. The interferometer 110 comprises an optical splitter 111, a1-symbol delay element 112, a π/4 phase shifter 113, and an opticalcoupler 114. In the interferometer 110, an optical signal arriving atthe optical coupler 114 from the optical splitter 111 via the 1-symboldelay element 112 interferes with an optical signal arriving at theoptical coupler 114 from the optical splitter 111 via the π/4 phaseshifter 113. The interferometer 110 generates a pair of complementaryoptical signals. In the same manner, the interferometer 120 comprises anoptical splitter 121, a 1-symbol delay element 122, a −π/4 phase shifter123, and an optical coupler 124, and generates a pair of complementaryoptical signals. Balanced photodiodes 131 and 132 convert the opticalsignals output from the corresponding interferometers 110 and 120 intoelectrical signals. The signals acquired from the balanced photodiodes131 and 132 are equivalent to the transmitted data.

A configuration and operation of the DQPSK optical receiver apparatusshown in FIG. 1 is described in, for example, Patent Document 1(Japanese publication of translated version No. 2004-516743(WO2002/051041 or US2004/0081470)) in detail.

FIG. 2 is a diagram showing another example of a conventional DQPSKoptical receiver apparatus. In FIG. 2, a pair of the optical signalsoutput from an optical splitter 101, are guided to optical splitters 141and 142. One of the output signals of the optical splitter 141 is guidedto an optical coupler 114 via a π/4 phase shifter 113, and the otheroutput signal of the optical splitter 141 is guided to an opticalcoupler 124. In the same way, one of the output signals of the opticalsplitter 142 is guided to an optical coupler 124 via a −π/4 phaseshifter 123, and the other output signal of the optical splitter 142 isguided to an optical coupler 114. At that time, the transmission timeperiod corresponding to an optical path from the optical splitter 101 tothe optical splitter 142 is longer than that corresponding to an opticalpath from the optical splitter 101 to the optical splitter 141 by thetime period of 1-symbol. Consequently, the 1-symbol delay elements 112and 122 shown in FIG. 1 are realized.

A configuration and operation of the DQPSK optical receiver apparatusshown in FIG. 2 is described in, for example, by Patent Document 2(WO2003/063515) in detail.

The amount of phase shift in the π/4 phase shifter 113 and the −π/4phase shifter 123 need to be adjusted with high precision in order tocontrol data error. For that reason, in optical receiver apparatus forreceiving high-speed data in particular, as shown in FIG. 2, adjusterunits 115 and 125 may be configured for adjusting the amount of phaseshift in the π/4 phase shifter 113 and the π/4 phase shifter 123. In thecase the amount of phase shift in the π/4 phase shifter 113 and the −π/4phase shifter 123 changes depending on the temperature, for example, theadjuster units 115 and 125 are heaters.

In Non-patent Document 1 (Michael Ohm, “Optical 8-DPSK and receiver withdirect detection and multilevel electrical signals”, Advanced ModulationFormats, 2004 IEEE/LEOS Workshop on 1-2 Jul. 2004, Pages 45-46.), forexample, it is described how 8-DPSK (or 2^(n)-DPSK where n is aninteger) optical signal can be received by configuring a multileveldetection circuit for processing an electrical signal output, convertedphotoelectrically by a DQPSK optical receiver apparatus, as a multilevelsignal. In addition, an optical signal modulated by DMAM (DifferentialM-ary Amplitude (shift keying) Modulation) such as a DQAM (DoubleQuadrature Amplitude Modulation) modulated signal can be received byusing a multilevel detection circuit, using a similar technique to thereception of the 8-DPSK optical signal, after photoelectric conversionto an electrical signal.

In the configuration shown in FIG. 1, two 1-symbol delay elements (112and 122) are required. Therefore, the configuration is not suitable forreducing the size of the optical receiver apparatus. Further, the1-symbol delay elements 112 and 122 must be adjusted so as to have thesame optical path length, and the adjustment is required in two lines.Therefore, the configuration is not favorable in terms of cost.

In the configuration shown in FIG. 2, the same function as that of theoptical receiver apparatus shown in FIG. 1 can be provided with one1-symbol delay element alone. However, in this configuration, the π/4phase shifter 113 and the −π/4 phase shifter 123 have to be locatedclose to each other in order to reduce the size of the optical receiverapparatus. For that reason, in an optical receiver apparatus comprisingheaters, coolers or electrodes for electro-optic effects etc. as theadjuster units 115 and 125, thermal or electrical crosstalk occurs dueto a spatial diffusion effect of the physical property, and thus thereis a possibility that the amount of phase shift of the π/4 phase shifter113 and the −π/4 phase shifter 123 can not be adjusted with highprecision.

SUMMARY OF THE INVENTION

It is an object of the present invention to reduce the size of adifferential multilevel optical signal receiver apparatus.

A differential multilevel optical signal receiver apparatus of thepresent invention comprises: a first optical splitter for splitting aninput differential multilevel modulated optical signal and forgenerating a first optical signal and a second optical signal; a secondoptical splitter for splitting the first optical signal and forgenerating a third optical signal and a fourth optical signal; a thirdoptical splitter for splitting the second optical signal and forgenerating a fifth optical signal and a sixth optical signal; a 1-symboldelay element configured between the first optical splitter and thethird optical splitter; a π/4 phase shifter element configured betweenthe first optical splitter and the third optical splitter; firstadjustment means, configured adjacent to the π/4 phase shifter element,for adjusting the amount of phase shift of the π/4 phase shifterelement; a first optical coupler for coupling the third optical signaland the fifth optical signal; a second optical coupler for coupling thefourth optical signal and the sixth optical signal; a π/2 phase shifterelement, configured between the second optical splitter and the firstoptical coupler or between the second optical splitter and the secondoptical coupler; second adjustment means, configured adjacent to the π/2phase shifter element, for adjusting the amount of phase shift of theπ/2 phase shifter element; and a photodetector circuit for convertingoptical signals output from the first and second optical couplers intoelectrical signals.

In the above configuration, the 1-symbol delay element is shared by apair of interferometers for demodulating a pair of modulated signals inthe differential multilevel modulated optical signal. The firstadjustment means for adjusting the amount of phase shift by the π/4phase shifter element is configured in the input side of the second andthird optical splitters, and the second adjustment means for adjustingthe amount of phase shift by the π/2 phase shift element is configuredin the output side of the second and third optical splitters. For thatreason, the first adjustment means and the second adjustment means arespatially separated in the configuration. Therefore, the amount of phaseshift of the π/4 phase shifter element is not affected by the secondadjustment means, and the amount of phase shift of the π/2 phase shifterelement is not affected by the first adjustment means.

In this configuration, it is also possible to configure the π/4 phaseshifter element closer to the input side than the 1-symbol delayelement, allowing further reduction of crosstalk.

The π/4 phase shifter element may be configured between the firstoptical splitter and the second optical splitter and the 1-symbol delayelement may be configured between the first optical splitter and thethird optical splitter. In such a case, the π/2 phase shifter element isconfigured between the third optical splitter and the first opticalcoupler, or between the third optical splitter and the second opticalcoupler.

According to the present invention, the number of the 1-symbol delayelements is reduced, and therefore the size of the differentialmultilevel optical signal receiver apparatus can be reduced. Even thoughthe differential multilevel optical signal receiver apparatus is reducedin size, the amount of phase shift of the π/4 phase shifter element andthe π/2 phase shifter element can be adjusted without being affected byeach other, and thus it is possible to reduce the size of the apparatuswithout causing a deterioration of reception quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram describing an example of a conventional DQPSKoptical receiver apparatus;

FIG. 2 is a diagram showing another example of a conventional DQPSKoptical receiver apparatus;

FIG. 3 is a diagram describing a configuration of an opticaltransmission system in which the DQPSK optical receiver apparatus of thepresent invention is configured;

FIG. 4 is a diagram describing a first configuration of the DQPSKoptical receiver apparatus of the present invention;

FIG. 5 is a diagram explaining the adjustment of the amount of phaseshift;

FIG. 6 is a diagram describing a second configuration of the DQPSKoptical receiver apparatus of the present invention;

FIG. 7 is a diagram describing a third configuration of the DQPSKoptical receiver apparatus of the present invention;

FIG. 8 is a diagram describing a fourth configuration of the DQPSKoptical receiver apparatus of the present invention;

FIG. 9 is a diagram showing a configuration of the DQPSK opticalreceiver apparatus comprising a function for adjusting a 1-symbol delayelement;

FIG. 10 is an embodiment of the DQPSK optical receiver apparatus of thefirst configuration;

FIG. 11 is an embodiment of the DQPSK optical receiver apparatus of thesecond configuration;

FIG. 12 is an embodiment of the DQPSK optical receiver apparatus of thethird configuration;

FIG. 13 is an embodiment of the DQPSK optical receiver apparatus of thefourth configuration;

FIG. 14 is a diagram describing a variation (1) of the configuration ofthe adjuster circuit;

FIG. 15 is a diagram describing a variation (2) of the configuration ofthe adjuster circuit;

FIG. 16 is an embodiment of the DQPSK optical receiver apparatus withits optical path realized by an optical fiber;

FIG. 17A and FIG. 17B are embodiments of another mode of the DQPSKoptical receiver apparatus of the present invention;

FIG. 18A and FIG. 18B are embodiments of reflection devices shown inFIG. 17A and FIG. 17B;

FIG. 19 is an example of a variation (1) of the first configuration; and

FIG. 20 is an example of a variation (2) of the first configuration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a diagram describing a configuration of an opticaltransmission system in which the DQPSK optical receiver apparatus of thepresent invention is used. The DQPSK optical receiver apparatus is onemode of a differential multilevel optical signal receiver apparatus.

In FIG. 3, a DQPSK optical transmitter apparatus 200 comprises a lightsource (for example, Laser Diode LD) 201, a π/2 phase shifter 202, andphase modulators 203 and 204. The light source 201 generates optical CW(Continuous Wave) output. The wavelength of the optical CW source is notlimited in particular; however, it is 1550 nm, for example. The π/2phase shifter 202 provides a phase difference of π/2 between a pair ofoptical CW inputs to the phase modulators 203 and 204. The phasemodulators 203 and 204 modulate the optical CW by data 1 and data 2,respectively. The data 1 and the data 2 here are bit streams generatedby encoding the transmitted data using a DQPSK pre-coder, not shown inthe drawings. A pair of the optical CW sources provided to the phasemodulators 203 and 204 have phases differing from each other by 90degrees. Therefore, when the optical signals generated by the phaseshifters 203 and 204 are combined, for example, “θ” “θ+π/2”, “θ+π” or“θ+3π/2” are assigned to each symbol “00”, “01”, “11” and “10”. TheDQPSK optical transmitter apparatus 200 transmits the DQPSK opticalsignal generated in the above manner.

The DQPSK optical signal is transmitted-via an optical fiber 210, and isreceived by a DQPSK optical receiver apparatus 300. The DQPSK opticalreceiver apparatus 300 comprises an optical input port 301, and theDQPSK optical signal is guided to a DQPSK optical receiver circuit viathe optical input port 301.

FIG. 4 is a diagram describing a first configuration of the DQPSKoptical receiver apparatus of the present invention. The DQPSK opticalreceiver apparatus receives the DQPSK optical signal via the opticalinput port 301.

An optical splitter 1 splits an input optical signal, and outputs thesignals to an optical path 11 and an optical path 12. Here, thesplitting ratio of the optical splitter 1 is 1:1, and thus, opticalsignals having equal optical power are transmitted via the optical path11 and the optical path 12. The optical path 11 is connected to anoptical splitter 2, and the optical path 12 is connected to an opticalsplitter 3.

The optical splitter 2 splits the optical signal provided via theoptical path 11, and outputs the signals to an optical path 13 and anoptical path 14. Here, the splitting ratio of the optical splitter 2 is1:1, and thus, optical signals having equal optical power aretransmitted via the optical path 13 and the optical path 14. The opticalpath 13 is connected to an optical coupler 4, and the optical path 14 isconnected to an optical coupler 5. In the same manner, the opticalsplitter 3 splits the optical signal provided via the optical path 12,and outputs the signals to an optical path 15 and an optical path 16.Here, the splitting ratio of the optical splitter 3 is 1:1, and thus,optical signals having equal optical power are transmitted via theoptical path 15 and the optical path 16. The optical path 15 isconnected to the optical coupler 4, and the optical path 16 is connectedto the optical coupler 5.

The optical path 12, which connects the optical splitter 1 and theoptical splitter 3, comprises a 1-symbol delay element 21 and a π/4phase shifter element 22. The 1-symbol delay element 21 is a part of theoptical path 12, and is a delay element for making the optical signalpropagation time from the optical splitter 1 to the optical splitter 3longer than the optical signal propagation time from the opticalsplitter 1 to the optical splitter 2 by “1-symbol time period”. The1-symbol delay element 21 can be realized, for example, by making theoptical path length of the optical path 12 longer than that of theoptical path 11 by “a length equivalent to 1-symbol time period”. Here,if it is assumed that the symbol rate of the DQPSK optical signal is 20G symbols/second, then the 1 symbol time period is 50 ps. Therefore,“the length equivalent to 1-symbol time period” is equivalent to thelength that the light is propagated in the optical path 12 within 50 ps.The optical propagation speed depends on the refractive index of anoptical path.

The π/4 phase shifter element 22 is a part of the optical path 12, andprovides a phase difference of π/4 (that is, π/4+nπ/2 where n is aninteger including zero) to a pair of optical signals transmitted via theoptical path 11 and the optical path 12. The π/4 phase shifter element22 is realized by adjusting the optical path length of the optical path12 using an adjuster circuit 23. If the wavelength λ of the carrier waveof the optical signal is 1550 nm, “the length λ/8” to acquire the phaseshift of π/4 is about 190 nm, and the physical length of thecorresponding optical path is about 130 nm under the condition that therefractive index n is 1.5.

The optical path 14, which connects the optical splitter 2 and theoptical coupler 5, comprises a π/2 phase shifter element 24. The π/2phase shifter element 24 is a part of the optical path 14, and providesa phase difference of π/2 (that is, π/2+nπ where n is an integerincluding zero) to a pair of optical signals transmitted via the opticalpath 13 and the optical path 14. The π/2 phase shifter element 24 isrealized by adjusting the optical path length of the optical path 14using an adjuster circuit 25. Under the same conditions as above, “thelength λ/4” to acquire the phase shift of π/2 is about 380 nm, and thephysical length of the corresponding optical path is about 260 nm.

Both the adjuster circuits 23 and 25 adjust the optical path length ofthe optical paths 12 and 14, utilizing the change in the volume andrefractive index of the optical path media with temperature. In such acase, the adjuster circuits 23 and 25 can be realized by for example, aheater utilizing electrical resistance, a Peltier effect element, or alight emitting element. The adjuster circuits 23 and 25 may be able toadjust the optical path length of the optical paths 12 and 14,respectively, using the changes in the refractive index with anelectro-optic effect or electron density change in a semiconductormaterial. In this case, the adjuster circuits 23 and 25 can be realizedby, for example, a circuit for adjusting the refractive index of theoptical paths 12 and 14 using the electro-optic effect. In either case,the adjuster circuit 23 is configured so as to be adjacent to the π/4phase shifter element 22 constituting a part of the optical path 12, andthe adjuster circuit 25 is configured so as to be adjacent to the π/2phase shifter element 24 constituting a part of the optical path 14.

The optical coupler 4 couples the optical signal transmitted via theoptical path 13 and the optical signal transmitted via the optical path15. These optical signals interfere with each other. The optical coupler4 outputs a pair of complementary optical signals. In the same manner,the optical coupler 5 couples the optical signal transmitted via theoptical path 14 and the optical signal transmitted via the optical path16, and outputs a pair of complementary optical signals.

A balanced photodiode (photodetector circuit) 131 comprises a pair ofphotodiodes, and converts a pair of optical signals output from theoptical coupler 4 into a pair of electrical signals. Then, thedifference between the pair of electrical signals is output. Similarly,The balanced photodiode (photodetector circuit) 132 converts a pair ofoptical signals output from the optical coupler 5 into a pair ofelectrical signals, and outputs the difference.

In the DQPSK optical receiver apparatus with the above configuration,when an optical path (a first arm) from the optical splitter 1 to theoptical coupler 4 via the optical splitter 2 is compared with an opticalpath (a second arm) from the optical splitter 1 to the optical coupler 4via the optical splitter 3, the second arm comprises the 1-symbol delayelement 21 and the π/4 phase shifter element 22. For that reason, theoptical signal arriving at the optical coupler 4 via the second arm,when compared with the optical signal arriving at the optical coupler 4via the first arm, is phase shifted by π/4, and is delayed by 1-symboltime period. Consequently, the interferometer comprising the first andthe second arms is equivalent to the interferometer 120 shown in FIG. 1.

When an optical path (a third arm) from the optical splitter 1 to theoptical coupler 5 via the optical splitter 2 is compared with an opticalpath (a fourth arm) from the optical splitter 1 to the optical coupler 5via the optical splitter 3, the third arm comprises the π/2 phaseshifter element 24, and the fourth arm comprises the 1-symbol delayelement 21 and the π/4 phase shifter element 22. For that reason, whencompared with the optical signal arriving at the optical coupler 5 viathe third arm, the optical signal arriving at the optical coupler 5 viathe fourth arm is phase shifted by −π/4, and is delayed by 1-symbol timeperiod. In other words, π/4 phase shift occurs in the third arm, and1-symbol delay occurs in the fourth arm. Therefore, the interferometercomprising the third and the fourth arms is equivalent to theinterferometer 110 shown in FIG. 1.

The configuration of the DQPSK optical receiver apparatus of theembodiment shown in FIG. 4 is equivalent to the DQPSK optical receiverapparatus shown in FIG. 1. Hence, the output signal of the balancedphotodiode 131 is equivalent to one of the data 1 or 2 beforepre-coding, and the output signal of the balanced photodiode 132 isequivalent to the other. The operation of the optical receiver apparatusshown in FIG. 1 is described in the above Patent Document 1, forexample.

The DQPSK optical receiver apparatus shown in FIG. 1 has a configurationcomprising two 1-symbol delay elements. However, the DQPSK opticalreceiver apparatus of the present embodiment provides equivalentfunctionality with only one 1-symbol delay element 21. Therefore, theDQPSK optical receiver apparatus of the present embodiment can besmaller in size, compared with the DQPSK optical receiver apparatusshown in FIG. 1.

The optical splitters 1-3 are not limited in particular, but can berealized by, for example, an optical directional coupler, an MMI(Multimode Interference) optical coupler, or a Y-split optical coupler.The optical couplers 4 and 5 are not particularly limited either;however, they can be realized by, for example, an optical directionalcoupler, an MMI optical coupler, or an X optical coupler.

The adjuster circuits 23 and 25 operate independently of each other. Atsuch a time, the adjuster circuits 23 and 25 may perform feedbackcontrol utilizing the output signals of the balanced photodiodes 131 and132. The feedback control can be realized by, for example, comprising amonitor circuit for monitoring the error rate of the output signal ofthe balanced photodiodes 131 and 132 and by adjusting the optical pathlength of the corresponding optical paths 12 and 14 so as to reduce (orminimize) the error rate.

FIG. 5 is a diagram explaining the adjustment of the amount of phaseshift. In this descriptions a feedback system in which the optical pathlength of the π/4 phase shifter element 22 (or the π/2 phase shifterelement 24) is adjusted utilizing thermal change is described. In such acase, the adjuster circuit 23 (or the adjuster circuit 25) is, forexample, a heater generating heat by applying a current to a resistance.In FIG. 5, a signal processing circuit 141 performs necessary processing(multiplexing, bit rearrangement etc.) of the output signal of thebalanced photodiodes 131 and 132, and recovers the data streamtransmitted from the transmitter apparatus. A monitor circuit 142monitors the bit error rate of the recovered data stream. Here, if theoptical path length of the π/4 phase shifter element 22 (or the π/2phase shifter element 24) (that is, the amount of phase shift) isadjusted properly, the error rate should be reduced. Therefore, themonitor circuit 142 generates an instruction to reduce the bit errorrate of the recovered data stream. A current control circuit 143, inaccordance with the instruction from the monitor circuit 142, controlsthe current passing through the adjuster circuit 23 (or the adjustercircuit 25). By so doing, the optical path length of the π/4 phaseshifter element 22 (or the π/2 phase shifter element 24) is optimized,and the bit error rate of the recovered data stream is reduced.

In the DQPSK optical receiver apparatus with the above configuration,the adjuster circuits 23 and 25 are separated from each other. Theadjuster circuit 23 is configured on the input side of the opticalsplitters 2 and 3, however the adjuster circuit 25 is configured on theoutput side of the optical splitters 2 and 3. Therefore, the control bythe adjuster circuit 23 (the control to adjust the temperature of theπ/4 phase shifter element 22, for example) hardly affects the π/2 phaseshifter element 24, and the control by the adjuster circuit 25 (thecontrol to adjust the temperature of the π/2 phase shifter element 24,for example) hardly affects the π/4 phase shifter element 22. As aresult, the amount of phase shift of the π/4 phase shifter element 22and the π/2 phase shifter element 24 is effectively independent and canbe adjusted with high precision, and it is possible to reduce the sizeof the DQPSK optical receiver apparatus, controlling the data error.

In the DQPSK optical receiver apparatus with the above configuration, itis desirable that the difference between the optical path length of theoptical path 11 and the optical path length which is the differencebetween the optical path length of the 1-symbol delay element 21 and theoptical path length of the optical path 12, is less than amultiplicative factor of 200 of the wavelength of the DQPSK opticalsignal. It is also desirable that the optical path lengths of theoptical paths 13-16 are approximately the same. In particular, thedifference between the optical path length of the optical path 13 andthe optical path length of the optical path 14 should be within amultiplicative factor of 200 of the wavelength of the DQPSK opticalsignal. In addition, the difference between the optical path length ofthe optical path 15 and the optical path length of the optical path 16should be also within a multiplicative factor of 200 of the wavelengthof the DQPSK optical signal. These relations are applicable not only tothe first configuration but also to the second through the fourthconfigurations explained later.

FIG. 6 is a diagram describing a second configuration of the DQPSKoptical receiver apparatus of the present invention. The secondconfiguration is basically the same as the first configuration. In thefirst configuration, the π/2 phase shifter element 24 is configured inthe optical path 14, which connects the optical splitter 2 and theoptical coupler 5. However, in the second configuration, the π/2 phaseshifter element 24 is configured in the optical path 13, which connectsthe optical splitter 2 and the optical coupler 4.

In the DQPSK optical receiver apparatus with the above configuration, anoptical path (a first arm) from the optical splitter 1 to the opticalcoupler 4 via the optical splitter 2 is compared with an optical path (asecond arm) from the optical splitter 1 to the optical coupler 4 via theoptical splitter 3, the first arm comprises the π/2 phase shifterelement 24, and the second arm comprises the 1-symbol delay element 21and the π/4 phase shifter element 22. For that reason, when comparedwith the optical signal arriving at the optical coupler 4 via the firstarm, the optical signal arriving at the optical coupler 4 via the secondarm is phase shifted by −π/4 and delayed by 1-symbol time period. Inother words, a π/4 phase shift occurs in the first arm, and a 1-symboldelay occurs in the second arm. Thus, the interferometer comprising thefirst and the second arms is equivalent to the interferometer 110 shownin FIG. 1.

When an optical path (a third arm) from the optical splitter 1 to theoptical coupler 5 via the optical splitter 2 is compared with an opticalpath (a fourth arm) from the optical splitter 1 to the optical coupler 5via the optical splitter 3, the fourth arm comprises the 1-symbol delayelement 21 and the π/4 phase shifter 22. For that reason, when comparedwith the optical signal arriving at the optical coupler 5 via the thirdarm, the optical signal arriving at the optical coupler 5 via the fourtharm is phase shifted by π/4, and delayed by 1-symbol time period. Inother words, a −π/4 phase shift occurs in the third arm, and a 1-symboldelay occurs in the fourth arm. Therefore, the interferometer comprisingthe third and the fourth arms is equivalent to the interferometer 120shown in FIG. 1.

As described above, the DQPSK optical receiver apparatus with the secondconfiguration is equivalent to the DQPSK optical receiver apparatusshown in FIG. 1. The same effect as that of the first configurationshown in FIG. 1 can also be obtained by the second configuration.

FIG. 7 is a diagram describing a third configuration of the DQPSKoptical receiver apparatus of the present invention. In the firstconfiguration, the π/4 phase shifter element 22 is configured in theoptical path 12, which connects the optical splitter 1 and the opticalsplitter 3, and the π/2 phase shifter element 24 is configured in theoptical path 14, which connects the optical splitter 2 and the opticalcoupler 5. In the third configuration, however, the π/4 phase shifterelement 22 is configured in the optical path 11, which connects theoptical splitter 1 and the optical splitter 2, and the π/2 phase shifterelement 24 is configured in the optical path 15, which connects theoptical splitter 3 and the optical coupler 4.

In the DQPSK optical receiver apparatus with the above configuration,when an optical path (a first arm) from the optical splitter 1 to theoptical coupler 4 via the optical splitter 2 is compared with an opticalpath (a second arm) from the optical splitter 1 to the optical coupler 4via the optical splitter 3, the first arm comprises the π/4 phaseshifter element 22, and the second arm comprises the 1-symbol delayelement 21 and the π/2 phase shifter element 24. For that reason, whencompared with the optical signal arriving at the optical coupler 4 viathe first arm, the optical signal arriving at the optical coupler 4 viathe second arm is phase shifted by π/4, and is delayed by 1-symbol timeperiod. In other words, a −π/4 phase shift occurs in the first arm, anda 1-symbol delay occurs in the second arm. Consequently, theinterferometer comprising the first and the second arms is equivalent tothe interferometer 120 shown in FIG. 1.

When an optical path (a third arm) from the optical splitter 1 to theoptical coupler 5 via the optical splitter 2 is compared with an opticalpath (a fourth arm) from the optical splitter 1 to the optical coupler 5via the optical splitter 3, the third arm comprises the π/4 phaseshifter element 22, and the fourth arm comprises the 1-symbol delayelement 21. For that reason, when compared with the optical signalarriving at the optical coupler 5 via the third arm, the optical signalarriving at the optical coupler 5 via the fourth arm is phase shifted by−π/4, and is delayed by 1-symbol time period. In other words, a π/4phase shift occurs in the third arm, and a 1-symbol delay occurs in thefourth arm. Therefore, the interferometer comprising the third and thefourth arms is equivalent to the interferometer 110 shown in FIG. 1.

As described above, the DQPSK optical receiver apparatus of the thirdconfiguration is equivalent to the DQPSK optical receiver apparatusshown in FIG. 1. In the third configuration, also, the same effect asthat of the first configuration shown in FIG. 4 can be obtained.

FIG. 8 is a diagram describing a fourth configuration of the DQPSKoptical receiver apparatus of the present invention. The fourthconfiguration is basically the same as the third configuration. In thethird configuration, the π/2 phase shifter element 24 is configured inthe optical path 15, which connects the optical splitter 3 and theoptical coupler 4. On the other hand, in the fourth configuration, theπ/2 phase shifter element 24 is configured in the optical path 16, whichconnects the optical splitter 3 and the optical coupler 5.

In the DQPSK optical receiver apparatus with the above configuration,when an optical path (a first arm) from the optical splitter 1 to theoptical coupler 4 via the optical splitter 2 is compared with an opticalpath (a second arm) from the optical splitter 1 to the optical coupler 4via the optical splitter 3, the first arm comprises the π/4 phaseshifter element 22, and the second arm comprises the 1-symbol delayelement 21. For that reason, when compared with the optical signalarriving at the optical coupler 4 via the first arm, the optical signalarriving at the optical coupler 4 via the second arm is phase shifted by−π/4, and is delayed by 1-symbol time period. In other words, a π/4phase shift occurs in the first arm, and a 1-symbol delay occurs in thesecond arm. Consequently, the interferometer comprising the first andthe second arms is equivalent to the interferometer 110 shown in FIG. 1.

When an optical path (a third arm) from the optical splitter 1 to theoptical coupler 5 via the optical splitter 2 is compared with an opticalpath (a fourth arm) from the optical splitter 1 to the optical coupler 5via the optical splitter 3, the third arm comprises the π/4 phaseshifter element 22, and the fourth arm comprises the 1-symbol delayelement 21 and the π/2 phase shifter element 24. For that reason, whencompared with the optical signal arriving at the optical coupler 5 viathe third arm, the optical signal arriving at the optical coupler 5 viathe fourth arm is phase shifted by π/4, and is delayed by 1-symbol timeperiod. In other words, a −π/4 phase shift occurs in the third arm, anda 1-symbol delay occurs in the fourth arm. Therefore, the interferometercomprising the third and the fourth arms is equivalent to theinterferometer 120 shown in FIG. 1.

As described above, the DQPSK optical receiver apparatus with the fourthconfiguration is equivalent to the DQPSK optical receiver apparatusshown in FIG. 1. In the fourth configuration, also, the same effect asthat of the first configuration shown in FIG. 4 can be obtained.

In the above embodiments, the adjuster circuits 23 and 25 adjust theamount of phase shift of the π/4 phase shifter element 22 and the π/2phase shifter element 24, respectively. The DQPSK optical receiverapparatus of the present embodiments may comprise an adjuster circuit 26for adjusting the optical path length of the 1-symbol delay element 21,as shown in FIG. 9. In such a case, the adjuster circuit 26 can berealized by, as in the adjuster circuits 23 and 25, a heater utilizingan electrical resistance, a Peltier effect element, or a light emittingelement, for example. Alternatively, the adjuster circuit 26 may adjustthe optical path length of the 1-symbol delay element 21 by utilizing achange in the refractive index. The adjuster circuit 26 can beconfigured in the DQPSK optical receiver apparatus with the firstthrough the fourth configurations. However, in any case, the adjustercircuit 26 is configured adjacent to the 1-symbol delay element 21.

FIG. 10 through FIG. 13 are the DQPSK optical receiver apparatus of thefirst through the fourth embodiments, respectively, of the presentinvention. In these embodiments, the optical splitters 1-3, the opticalcouplers 4 and 5, and the optical paths 11-16 are realized by atwo-dimensional optical waveguide circuit formed on the upper surface ofan optical waveguide substrate 30. The adjuster circuits 23 and 25 areconfigured on the optical waveguide substrate 30. Additionally, anoptical waveguide 17 to which the DQPSK optical signal is incident isformed on a prescribed end (input side end) of the optical waveguidesubstrate 30. And, the optical waveguides 18 a-18 d, which transmit theoutput signal of the optical couplers 4 and 5, are formed at the otherend (end different from the input side end) of the optical waveguidesubstrate 30. In other words, the optical input port 301 and thebalanced photodiodes 131 and 132 are configured on separate side ends ofthe optical waveguide substrate 30.

In the case that the DQPSK optical receiver apparatus of the presentembodiments is realized by a two-dimensional optical waveguide circuit,the optical paths 14 and 15 intersect with each other on one plane.However, a technology to avoid the interference of optical signalstransmitted via two intersecting optical waveguides has been knownheretofore (for example, see Japanese laid-open unexamined patentpublication No. 2001-343542, Japanese laid-open unexamined patentpublication No. 57-88410, and Japanese Patent No. 3201554).

As described above, when the DQPSK optical receiver apparatus isrealized by a two-dimensional optical waveguide circuit, it is possibleto reduce the size of the apparatus.

FIG. 14 and FIG. 15 are diagrams describing variations of theconfiguration of the adjuster circuit 25. In an example shown in FIG.14, the adjuster circuit 25 is configured in a region, which includesthe intersection of the optical path 14 and the optical path 15.According to this configuration, the configuration of the adjustercircuit 25 is simple. In an example shown in FIG. 15, the adjustercircuit 25 (25 a and 25 b) is divided so that it is configured in tworegions, which do not include the intersection. According to thisconfiguration, the control by the adjuster circuit 25 (“heat control”when the adjuster circuit 25 is a heater) does not affect the opticalpath 15, and thus, improvement of the adjustment precision is expected.In the region, which does not include the intersection of the opticalpath 14 and the optical path 15, not shown in the drawings inparticular, an undivided adjuster circuit 25 may be configured.

In the DQPSK optical receiver apparatus of the present embodiments, eachof the optical paths 11-16 can be configured by an optical fiber, asshown in FIG. 16. In such a case, it is desirable to use a single modeoptical fiber. Alternatively, a polarization maintaining single modeoptical fiber may be used. An optical directional coupler, a multimodeinterference optical coupler, a Y-split optical coupler etc. forexample, can be used as the optical splitters 1-3 and the opticalcouplers 4 and 5.

FIG. 17A and FIG. 17B are embodiments of another aspect of the DQPSKoptical receiver apparatus of the present invention. FIG. 17A is aschematic diagram showing a top view of the propagation of an opticalsignal in the optical receiver apparatus. FIG. 17B is a schematicdiagram showing an oblique perspective view of the propagation of theoptical signal in the optical receiver apparatus. FIG. 17A and FIG. 17Bdescribe the same optical receiver apparatus.

In FIG. 17A and FIG. 17B, the input DQPSK optical signal is directed toa half mirror 42 via a lens 41. The optical beam a, reflected by thehalf mirror 42, is directed to a reflection device 43. The reflectiondevice (optical beam shift and half mirror) 43, comprising a mirror 43a, a half mirror 43 b, and a mirror 43 c as shown in FIG. 18A, generatesa pair of parallel optical beams b and c, and directs the beams to amirror 45. In other words, the optical beam reflected by the mirror 43 ais split by the half mirror 43 b. The optical beam b, which passedthrough the half mirror 43 b, is reflected by the mirror 43 c, and then,is directed to the mirror 45. The optical beam c, reflected by the halfmirror 43 b, is also directed to the mirror 45. However, the opticalbeam d, which passed through the half mirror 42, is directed to themirror 46.

Here, the optical path length of the optical path from the half mirror42 to the mirror 45 via the reflection device 43 is ΔL longer than theoptical path length of the optical path from the half mirror 42 to themirror 46. The ΔL is equivalent to the distance that an optical beam ispropagated in 1-symbol time period. By this means a 1-symbol delayelement is achieved. A π/4 phase shifter element 44 is configured in theoptical path from the half mirror 42 to the mirror 45 via the reflectiondevice 43. The π/4 phase shifter element 44 is realized by adjusting theoptical path length of the optical path from the half mirror 42 to themirror 45 via the mirror 43.

A pair of optical beams e and f reflected by the mirror 45 is split by ahalf mirror 48. The optical beam g reflected by the mirror 46 isdirected to a reflection device 47. The configuration of the reflectiondevice 47 is basically the same as that of the reflection device 43,comprising a mirror 47 a, a half mirror 47 b and a mirror 47 c as shownin FIG. 18B, and it generates a pair of parallel optical beams h and i.The pair of optical beams h and i is split by the half mirror 48. A π/2phase shifter element 49 is configured in one of the pair of opticalpaths (the optical path propagating the optical beam i) from the mirror46 to the half mirror 48. The π/2 phase shifter element 49 is realizedby adjusting the optical path length of the optical path from the mirror46 to the half mirror 48.

In the half mirror 48, a pair of optical beams j and k is obtained fromthe interference between the optical beams e and h. The optical beam jis directed to one of the photodiodes in the balanced photodiode 131 viaa mirror 50 and a condenser lens 52, and the optical beam k is directedto the other photodiode of the balanced photodiode 131 via a mirror 51and a condenser lens 53. In the same manner, a pair of optical beams mand n is obtained from the interference between the optical beams f andi. The optical beam m is directed to one of the photodiodes in thebalanced photodiode 132 via the mirror 50 and the condenser lens 52, andthe optical beam n is directed to the other photodiode in the balancedphotodiode 132 via the mirror 51 and the condenser lens 53.

In the above configuration, the optical splitter 1 shown in FIG. 4 isequivalent to the half mirror 42. The optical splitter 2 is equivalentto the reflection device 47. The optical splitter 3 is equivalent to thereflection device 47. The optical couplers 4 and 5 are equivalent to thehalf mirror 48. Each of the mirrors and half mirrors is a mirror whichdoes not provide a phase difference between the p-polarization ands-polarization.

In the DQPSK optical receiver apparatus with the above firstconfiguration and the second configuration, the optical splitters 1 and2 can be replaced by one optical device. In such a case, the opticalsplitters 1 and 2 can be replaced by a multimode interference coupler ora 1:3 optical coupler etc., for example.

FIG. 19 and FIG. 20 are examples of variations of the firstconfiguration shown in FIG. 4. In the configuration shown in FIG. 19,the optical splitter 3 is configured within the 1-symbol delay element21. In such a case, the 1-symbol delay element 21 is Y-shaped. The1-symbol delay element 21 is formed so that the propagation time of theoptical path from the optical splitter 1 to the optical coupler 4 viathe optical splitter 3 is 1-symbol time period longer than that of theoptical path from the optical splitter 1 to the optical coupler 4 viathe optical splitter 2, and that the propagation time of the opticalpath from the optical splitter 1 to the optical coupler 5 via theoptical splitter 3 is 1-symbol time period longer than that of theoptical path from the optical splitter 1 to the optical coupler 5 viathe optical splitter 2.

In the configuration shown in FIG. 20, the optical splitter 3 isconfigured immediately after the π/4 delay element 22, and 1-symboldelay elements are configured in parallel in the later stage. In such acase, also, the 1-symbol delay element 21 is formed so that thepropagation time of the optical path from the optical splitter 1 to theoptical coupler 4 via the optical splitter 3 is 1-symbol time periodlonger than the propagation time of the optical path from the opticalsplitter 1 to the optical coupler 4 via the optical splitter 2, and thepropagation time of the optical path from the optical splitter 1. to theoptical coupler 5 via the optical splitter 3 is 1-symbol time periodlonger than the propagation time of the optical path of the opticalsplitter 1 to the optical coupler 5 via the optical splitter 2.

1. (canceled)
 2. A differential multilevel optical signal receiverapparatus, comprising: a first optical splitter for splitting an inputdifferential multilevel optical signal and for generating a firstoptical signal and a second optical signal; a second optical splitterfor splitting the first optical signal and for generating a thirdoptical signal and a fourth optical signal; a third optical splitter forsplitting the second optical signal and for generating a fifth opticalsignal and a sixth optical signal; a π/4 phase shifter elementconfigured between the first optical splitter and the second opticalsplitter; first adjustment means, configured adjacent to the π/4 phaseshifter element, for adjusting the amount of phase shift of the π/4phase shifter element; a 1-symbol delay element configured between thefirst optical splitter and the third optical splitter; a first opticalcoupler for coupling the third optical signal and the fifth opticalsignal; a second optical coupler for coupling the fourth optical signaland the sixth optical signal; a π/2 phase shifter element, configuredbetween the third optical splitter and the first optical coupler, orbetween the third optical splitter and the second optical coupler;second adjustment means, configured adjacent to the π/2 phase shifterelement, for adjusting the amount of phase shift of the π/2 phaseshifter element; and a photodetector circuit for converting opticalsignals output from the first and second optical couplers intoelectrical signals. 3-22. (canceled)
 23. A differential multileveloptical signal receiver apparatus, comprising: a first optical devicefor splitting an input differential multilevel optical signal and forgenerating a first optical signal and a second optical signal; a secondoptical device for splitting the first optical signal and for generatinga third optical signal and a fourth optical signal; a third opticaldevice for splitting the second optical signal and for generating afifth optical signal and a sixth optical signal; a fourth optical devicefor coupling the third optical signal and the fifth optical signal andfor coupling the fourth optical device and the sixth optical device; aπ/4 phase shifter element for shifting the phase of the second opticalsignal by π/4; first adjustment means, configured adjacent to the π/4phase shifter element, for adjusting the amount of phase shift of theπ/4 phase shifter element; a 1-symbol delay element comprising a part ofan optical path configured between the first optical device and thethird optical device and a part of an optical path configured betweenthe third optical device and the fourth optical device; a π/2 phaseshifter element for shifting the phase of the fourth optical signal byπ/2; second adjustment means, configured adjacent to the π/2 phaseshifter element, for adjusting the amount of phase shift of the π/2phase shifter element; and a photodetector circuit for converting anoptical signal output from the fourth optical device into an electricalsignal, wherein the 1-symbol delay element makes the propagation time ofan optical path from the first optical device to the fourth opticaldevice via the third optical device 1-symbol time period longer than thepropagation time of an optical path from the first optical device to thefourth optical device via the second optical device.
 24. A differentialmultilevel optical signal receiver apparatus, comprising: a firstoptical device for splitting an input differential multilevel opticalsignal and for generating a first optical signal and a second opticalsignal; a second optical device for splitting the first optical signaland for generating a third optical signal and a fourth optical signal; athird optical device for splitting the second optical signal and forgenerating a fifth optical signal and a sixth optical signal; a fourthoptical device for coupling the third optical signal and the fifthoptical signal and for coupling the fourth optical device and the sixthoptical device; a π/4 phase shifter element for shifting the phase ofthe second optical signal by π/4; first adjustment means, configuredadjacent to the π/4 phase shifter element, for adjusting the amount ofphase shift of the π/4 phase shifter element; a 1-symbol delay elementfor delaying each of the fifth and the sixth optical signals by 1-symboltime period; a π/2 phase shifter element for shifting the phase of thefourth optical signal by π/2; second adjustment means, configuredadjacent to the π/2 phase shifter element, for adjusting the amount ofphase shift of the π/2 phase shifter element; and a photodetectorcircuit for converting an optical signal output from the fourth opticaldevice into an electrical signal.
 25. (canceled)